1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a recording material by using an electrophotographic process, for example, an image forming apparatus such as a copier, a printer, or a facsimile.
2. Related Background Art
Conventionally, there have been suggested or worked various image forming apparatuses using an electrophotographic process. For example, there are an image forming apparatus in which toner images formed on a photosensitive drum are sequentially transferred and superposed on top of one another on a recording material which is conveyed with being borne by a transfer drum or by a transfer belt and an image forming apparatus in which toner images formed on a photosensitive drum are primarily transferred and sequentially superposed on top of one another on an intermediate transfer drum or on an intermediate transfer belt and then the toner images on the intermediate transfer drum or on the intermediate transfer belt are secondarily transferred to a recording material. The image forming apparatuses using these two methods will be described below.
Simply describing the image forming process of an image forming apparatus in which the former method (a transfer belt) is used, first a photosensitive drum rotates so as to be uniformly charged on its surface by a charger. Next, the photosensitive drum is irradiated with a laser beam modulated by an image signal of a first color, for example, magenta of an original so as to form an electrostatic latent image of magenta on the photosensitive drum. The electrostatic latent image is developed by a magenta developing unit so as to form a magenta toner image of the first color on the photosensitive drum.
On the other hand, the recording material in a sheet feeding cassette is conveyed to the transfer belt by a registration roller or the like. Simultaneously with this recording material conveyance, an adsorbing roller is pressed to a surface of the transfer belt so that the transfer belt is charged by an adsorbing charger from a back side of the transfer belt in order to adsorb the recording material electrostatically onto the transfer belt. This transfer belt is rotating in synchronism with the photosensitive drum and the magenta toner image formed on the photosensitive drum by the transfer charger is transferred to the recording material borne by the transfer belt. The transfer belt continues its rotation without change to prepare for a transfer of a cyan toner image of the subsequent second color.
Next, the cyan toner image of the second color is formed on the photosensitive drum and the cyan toner image is transferred and superposed on the magenta toner image on the recording material borne by the transfer belt. The same image forming process is repeated also for the third and fourth colors yellow and black to obtain a full color image in which four-colored, magenta, cyan, yellow, and black toner images are superposed on each other on the recording material.
The recording material to which the four-colored toner images have been transferred is separated from the transfer belt and conveyed to a fixing device. The fixing device heats and presses the toner image and the recording material by a fixing roller and a pressure roller to mix respective colors of the toner image and to fix it to the recording material, by which a full-color print image is formed and then the recording material is discharged to an outside of the apparatus.
Briefly describing the image forming process of an image forming apparatus using the latter method (an intermediate transfer belt), a photosensitive drum is driven to rotate at a predetermined peripheral speed so as to be uniformly charged on its surface by a charger and is exposed to a laser beam with scanning by an exposing device, by which an electrostatic latent image of a first color is formed on the photosensitive drum and then the latent image is developed by a developing device. The developing device contains four developing units for yellow toner, magenta toner, cyan toner, and black toner, respectively. The electrostatic latent image of the first color on the photosensitive drum is developed by a yellow developing unit so as to be visualized as a yellow toner image.
The formed yellow toner image is electrostatically transferred to an intermediate transfer belt in a primary transfer portion where the intermediate transfer belt is put in contact with the photosensitive drum (a primary transfer). Toner remaining on the photosensitive drum which has been completed to be primarily transferred is removed from its surface by a cleaner and then the photosensitive drum is supplied to the next color image formation.
In the same manner, the photosensitive drum is charged by the charger and is exposed to a laser beam to form a second color electrostatic latent image, and then the latent image on the photosensitive drum is developed by a magenta developing unit to form a magenta toner image on the photosensitive drum. The magenta toner image is transferred and superposed on the yellow toner image on the intermediate transfer belt.
The above process is repeated also for cyan and black and respective toner images are sequentially superposed on the intermediate transfer belt for transfers. Thereby a color image is formed with four-colored, yellow, magenta, cyan, and black toner images laminated on the intermediate transfer belt.
Afterward a secondary transfer charger which has been spaced from the intermediate transfer belt is made to abut against a surface of the intermediate transfer belt, and in a secondary transfer portion where the intermediate transfer belt is in contact with the secondary transfer charger, toner images of the four colors on the intermediate transfer belt are collectively transferred to a surface of a recording material conveyed at a predetermined timing (a secondary transfer).
The recording material to which the toner images of the four colors have been transferred is conveyed from the intermediate transfer belt to a fixing device, where it is subjected to a fixing process with a heat roller or the like so as to make a full-color permanent image and then discharged to an outside of the image forming apparatus.
In the transfer process of the image forming apparatus in the former and the primary transfer process and the secondary transfer process of the image forming apparatus in the latter, a constant current power supply is connected to the transfer charger, the primary transfer charger, and the secondary transfer charger to control the transfer current at constant current from a viewpoint of a stability of the image transfer.
In the former image forming apparatus, however, poor transferring may occur as described below.
In this image forming apparatus, an image can be formed on a recording material having a smaller size than the applicable maximum size (in a length in a direction perpendicular to a recording material conveying direction). At this point, a volume resistivity of the recording material varies within a range of approx. 2xc3x97107 to 1014 xcexa9cm according to a type of the recording material or hygroscopic conditions.
If a recording material having the applicable maximum size is passed for a transfer by using the transfer belt as shown in FIG. 12, assuming that Itr[xcexcA] is transfer current from the transfer charger, "khgr"[cm] is a width of the recording material (a length in a thrust direction perpendicular to a transfer belt moving direction), and v[cm/s] is a rotating speed of the photosensitive drum, that is, a process speed, the transfer current flowing in xcex94t sec is Itrxc3x97xcex94t and a target area for the recording material is vxc3x97xcex94txc3x97"khgr" in this condition as shown in FIG. 13, and therefore the surface charge density on the recording material is expressed as follows:
Itrxc3x97xcex94t/("khgr"xc3x97xcex94txc3x97v)=Itr/("khgr"xc2x7v)[xcexcC/cm2]
While the recording material has a relatively very high volume resistivity in an ordinary temperature and low humidity environment (for example, 23xc2x0 C., 5% RH), a surface charge density on the maximum-sized recording material having the maximum width does not change in the constant current method even in such a condition.
On the other hand, if a small-sized recording material having a short width is passed as shown in FIG. 14, assuming that I"khgr" is electric current flowing through a part of a recording material, Iw and Iz are electric current flowing through no recording material passing portions in both sides of the recording material, respectively, "khgr" is a width of the recording material, and w and z are width of no recording material passing portions, respectively, a recording material passing portion has a resistance of the recording material itself though the surface charge density on the recording material and on the no recording material passing portion must be intrinsically the same as that in passing the maximum-sized recording material (in other words, Iw/(wxc2x7v)=I"khgr"/("khgr"xc2x7v)=Iz/(zxc2x7v)) and therefore its impedance is relatively high in comparison with the no recording material passing portion, by which electric current per unit area flowing through the recording material passing portion is smaller than that of the no recording material passing portion and also a surface charge density on the recording material becomes smaller than that of the no recording material passing portion.
Therefore, in the constant current method, the surface charge density on the recording material does not satisfy a necessary and sufficient condition (in other words, Iw/(wxc2x7v)=Iz/(zxc2x7v) greater than I"khgr"/("khgr"xc2x7v)), thus causing poor transferring.
It remarkably occurs, for example, when an image is transferred to a recording material having a high resistance increased due to dryness or when the transfer belt has a low resistance decreased due to moisture absorption in a high-temperature and high-humidity environment (for example, 30xc2x0 C., 80% RH).
A surface potential of a portion corresponding to the no recording material passing portion of the photosensitive drum is normally the same as a potential (Vd) of a no image portion which is a high-potential portion of the photosensitive drum as shown in FIG. 15 in order to prevent toner from adhering to the surface in developing. In this condition, if a small-sized recording material is passed in this constant current method, a lot of electric current flows into the no recording material passing portion having a large difference of potentials and electric current flowing through the recording material passing portion becomes insufficient, thus causing poor transferring.
Accordingly if a transfer bias is set in such a way that sufficient electric current flows into the recording material passing portion when a small-sized recording material is passed, to the contrary, excess electric current flows on passing a recording material having the maximum width, still causing poor transferring due to inversely charging of toner.
Also in the latter image forming apparatus, there have been problems in the primary transfer and the secondary transfer as described below when an image is formed on a recording material such as a postcard or an envelope having a smaller size in width than the maximum size.
First, problems in the primary transfer will be described below. A schematic diagram of the primary transfer portion is shown in FIG. 24.
In general, an impedance of an image portion is higher than that of a no image portion in the primary transfer since toner itself has a high resistance (approx. 1015 xcexa9cm). Therefore in a constant current control, as an image ratio (assuming that y is a length in a thrust direction of the image portion and g is the maximum size recording width, the image ratio (%)=y/gxc3x97100, 0xe2x89xa6yxe2x89xa6g) is lowered, a ratio of transfer current flowing into the image portion (toner portion) is decreased as shown in FIG. 25 and a ratio of transfer current flowing into the no image portion is increased. It causes a problem that a primary transfer efficiency decreases. This problem remarkably occurs in forming an image on a recording material such as a postcard or an envelope having a smaller size in width than the maximum size recording width.
Referring to FIG. 26, there is shown a state of the primary transfer portion when an image is formed onto a small-sized sheet. In FIG. 26 assuming that y is a length in the thrust direction of the image portion, g is the maximum size recording width, and a is a small-sized sheet width, 0xe2x89xa6yxe2x89xa6a less than g and therefore the image ratio (y/gxc3x97100) is always low. In other words, when an image is formed on a small-sized sheet, an image is not formed on a portion (gxe2x88x92a) corresponding to a no recording material passing portion in the primary transfer portion and therefore an image ratio is always low, thereby transfer current is always insufficient.
Furthermore, a surface potential of the photosensitive drum corresponding to a no recording material passing portion of the intermediate transfer belt is normally equal to a potential (vd) of the no image portion which is a high potential portion as shown in FIG. 27 so as to prevent toner from adhering to the surface in developing. Thereby in the constant current method, a large amount of electric current flows through the no recording material passing portion and the no image portion having a large potential difference at the primary transfer and electric current flowing through the image portion becomes insufficient, thereby causing poor transferring.
If a primary transfer bias is set in such a way that sufficient electric current flows through the image portion when an image is formed on a small-sized recording material to the contrary from this viewpoint, excess electric current flows in the primary transfer portion when an image is formed on a recording material having the maximum width this time and toner is inversely charged, thereby causing poor primary transferring. In addition if the primary transfer bias is controlled at a constant voltage, a very high transfer voltage is required to feed the image portion with necessary and sufficient transfer current and electric current flowing into the no image portion significantly increases, thereby being attended by an evil causing a memory phenomenon on the photosensitive drum.
Problems in the secondary transfer will be described below. In the secondary transfer portion, a constant current method is generally used as described above. The constant current method, however, has problems described below.
Generally this type of an apparatus normally treats recording materials having a smaller size than the applicable maximum size. In addition, a volume resistivity of recording materials varies within a range of approx. 2xc3x97107 to 1014 xcexa9cm according to a type of the recording material or hygroscopic conditions.
As shown in FIG. 28, if a recording material having the applicable maximum size is passed by the intermediate transfer belt for the secondary transfer, assuming that Itr [xcexcA] is transfer current from a transfer charger, "khgr"[cm] is a width of a recording material (a width in a thrust direction perpendicular to an intermediate transfer belt moving direction), and v[cm/s] is a rotating speed of the photosensitive drum, that is, a process speed, the transfer current flowing in xcex94t sec is Itrxc3x97xcex94t and a target area of the recording material is vxc3x97xcex94txc3x97"khgr" in this condition as shown in FIG. 29, and therefore the surface charge density on the recording material is expressed as follows:
Itrxc3x97xcex94t/("khgr"xc3x97xcex94txc3x97v)=Itr/("khgr"xc2x7v)[xcexcC/cm2]
While the recording material has a very high volume resistivity in an ordinary temperature and low humidity environment (for example, 23xc2x0 C., 5% RH), a surface charge density on the maximum-sized recording material does not change in the constant current method even in such a condition.
On the other hand, if a small-sized recording material is passed as shown in FIG. 30, assuming that I"khgr" is electric current flowing through a part of the recording material, Iw and Iz are electric current flowing through no recording material passing portions in both sides of the recording material, respectively, "khgr" is a width of the recording material, and w and z are widths of the no recording material passing portions, respectively, a recording material passing portion has a resistance of the recording material itself though the surface charge density on the recording material and on the no recording material passing portion must be intrinsically the same as that in passing the maximum-sized recording material (in other words, Iw/(wxc2x7v)=I"khgr"/("khgr"xc2x7v)=Iz/(zxc2x7v)) and therefore its impedance is relatively high in comparison with the no recording material passing portion, by which electric current per unit area flowing through the recording material passing portion is smaller than that of the no recording material passing portion and also a surface charge density on the recording material is smaller than that of the no recording material passing portion.
Therefore, in the constant current method, the surface charge density on the recording material does not satisfy a necessary and sufficient condition (in other words, Iw/(wxc2x7v)=Iz/(zxc2x7v) greater than I"khgr"/("khgr"xc2x7v)), thus causing poor transferring.
It remarkably occurs, for example, when an image is transferred to a recording material having a high resistance increased due to dryness or when the intermediate transfer belt has a low resistance decreased due to moisture absorption in a high-temperature and high-humidity environment (for example, 30xc2x0 C., 80%RH).
Therefore it is an object of the present invention to provide an image forming apparatus capable of preventing poor transferring from occurring when an image is transferred from an image bearing member to a recording material borne by a recording material bearing member.
It is another object of the present invention to provide an image forming apparatus capable of preventing poor transferring from occurring when an image is transferred from an image bearing member to an intermediate transfer member.
It is still another object of the present invention to provide an image forming apparatus capable of preventing poor transferring from occurring when an image is transferred from an intermediate transfer member to a recording material.
These and other objects, features and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.